As Ellen said, your horse's head carriage likely won't be fixed with a stronger bit because head carriage should be a natural reaction to having the rest of the body working correctly. A pelham, in particular, seems to be more of a way to hide your problem, than to fix it.
I'd start trying to figure out what it is about the bit she finds uncomfortable. Is her teeth up to date? Does she have a large tongue? Has she got a small mouth? Is the bit chaffing anywhere? Has she got room for the curb part of that bit? Does she prefer more to play with in her mouth so would prefer more joints? Does she prefer less to play with and so would prefer a single joint or a straight bar? Would she prefer rubber, or sweet mouth, or copper? Is it definitely the bit? Maybe she's carrying her head wrong to avoid pain in her back or neck? Maybe the nose band is too tight or pinching her? Maybe it's just schooling since she is young?
Some saddlers are trained in bridlery too. It might be worth enlisting one to help. It's not a case of whether a pelham will not work either, it might suit your horse great. However, picking a pelham just because your horse has poor head carriage and it will force your horse into more of an "outline"* is not the way to go.
*"Outline" here being in inverted commas because it would be a false outline, causing your horse to become weak through the back and sore. False outlines are very common and will lead to problems in the future, such as pain, sourness, or even breakdown, depending on the horse.
If she’s not comfortable in her current bit maybe try a different snaffle before jumping to the pelham. Correct head carriage doesn’t come from the bit but from the horse working correctly through their body
she has no head carriage and i dont think she is quite comfortable in her current bit (d ring broken segunda snaffle)
If she is going well in her current bit, don’t change it. I know its tempting to get a pelham for the equitation but don’t get one unless you feel that you need the extra leverage. But since she’s so young, I would try getting her in an eq-type frame in a snaffle before getting a leverage bit
It is better to start with a simple one, then you will adapt it